A person's gait describes the characteristics of their walking behaviour. More formally, gait is defined as the way locomotion is achieved through use of the legs. While gait appears similar from person to person, there are in fact a number of differences between individuals, so much so that a person's gait can be considered unique, with gait features being used for security applications. Common examples of gait features include walking speed, cadence, step size and body sway, but gait can also include minor features such as the angle of the toes.
Walking is a complex operation, involving the musculoskeletal system, processing of sensory information, coordination and balance. If one or more of these systems becomes compromised, this will likely be reflected in changes to a person's gait. It is therefore not surprising that gait can be related to a person's health. Some studies have found strong correlations between walking speed and becoming functionally dependent in the future. Gait has also been linked to cognitive decline such as Alzheimer's disease.
Gait features can be measured with high accuracy with apparatus such as walking mats and motion capture cameras, but can also be approximated using more portable apparatus, such as those comprising accelerometers which can be placed at various points on the body. An accelerometer measures the acceleration occurring during the movement of the user (and also the force of gravity) along three orthogonal measurement axes, and the measurements can be processed to identify the vertical, forward and lateral components of acceleration experienced by the user. The forward component of acceleration can be used to estimate walking speed, while step length can be estimated through the use of an ‘inverted pendulum’ model, which also makes use of vertical displacement.
Ideally, the accelerometer is carried or worn by the user in such a way that the orientation of the accelerometer with respect to the user does not change during movement, and the orientation of the accelerometer with respect to the user is known so that the measurements along the measurement axes can be transformed (i.e. rotated) to the reference frame of the user (i.e. the vertical, forward and lateral directions). Even more ideally, the orientation of the accelerometer with respect to the user is such that the measurement axes of the accelerometer align with the vertical, forward and lateral directions of the user, avoiding the need for a transformation to be performed.
In practice though, the three accelerometer axes will rarely be precisely aligned with the vertical, lateral and forward directions. This is especially true where the accelerometer is not fixedly attached to the user's body, e.g. those carried in pockets or worn as a pendant.
It is possible to estimate the vertical component of the acceleration signal, making use of the constant acceleration caused by gravity, but it is difficult to tell the difference between the lateral and forward accelerations without knowing the orientation of the accelerometer. Rotations along the accelerometer's measurement axes cause no measureable acceleration, so knowing the orientation of the accelerometer is difficult without the aid of additional sensors, such as gyroscopes and/or magnetometers, which are both sensors with comparatively high power requirements.
US 2010/0161271 describes techniques for determining orientation of a three-axis accelerometer in which acceleration due to gravity is measured on each axis x, y, z of the accelerometer and the direction of gravity is used to associate or align the x-axis of the accelerometer with gravity, the acceleration not due to gravity is then used to identify the forward motion and to associate or align the forward direction with the y-axis. The remaining direction may be identified as the sideways direction, which may be associated or aligned with the z-axis. The forward motion can be identified as the accelerometer axis that has the greatest energy.
A disadvantage with the technique in US 2010/0161271 is that it is based on the assumption that the (non-vertical) component with the highest energy is the forward component. However, this assumption only holds for normal walking speeds (e.g. 5 km/h and above). At lower walking speeds, the energy of the forward and lateral components become increasingly similar, which makes it difficult, if not impossible, to distinguish the components using this technique.
Therefore there is a need for an alternative technique for determining the lateral component of acceleration from acceleration measurements.